Bon, en fait, il semble que sendmail galère un peu...
Mon fichier mail.log ressemble à ça :
May 3 10:32:23 koffee-X406UAR sendmail[19943]: x438WNMS019943: from=www-data, size=410, class=0, nrcpts=0, msgid=<201905030832.x438WNMS019943@koffee-X406UAR>, relay=www-data@localhost
May 3 10:37:07 koffee-X406UAR sendmail[19580]: x438IAqO019580: from=koffee, size=0, class=0, nrcpts=0, msgid=<201905030818.x438IAqO019580@koffee-X406UAR>, relay=koffee@localhost
May 3 10:37:07 koffee-X406UAR sendmail[19562]: x438GqU5019562: from=koffee, size=0, class=0, nrcpts=0, msgid=<201905030816.x438GqU5019562@koffee-X406UAR>, relay=koffee@localhost
May 3 10:37:07 koffee-X406UAR sendmail[19561]: x438GlZt019561: from=koffee, size=0, class=0, nrcpts=0, msgid=<201905030816.x438GlZt019561@koffee-X406UAR>, relay=koffee@localhost
May 3 10:37:07 koffee-X406UAR sendmail[19579]: x438I6bw019579: from=koffee, size=0, class=0, nrcpts=0, msgid=<201905030818.x438I6bw019579@koffee-X406UAR>, relay=koffee@localhost
May 3 10:37:07 koffee-X406UAR sendmail[19566]: x438HW4N019566: from=koffee, size=0, class=0, nrcpts=0, msgid=<201905030817.x438HW4N019566@koffee-X406UAR>, relay=koffee@localhost
May 3 10:37:47 koffee-X406UAR sm-mta[1061]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 10:37:49 koffee-X406UAR sm-msp-queue[1166]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 10:38:47 koffee-X406UAR sm-mta[1061]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 10:38:47 koffee-X406UAR sm-mta[1173]: starting daemon (8.15.2): SMTP+queueing@00:10:00
May 3 10:38:49 koffee-X406UAR sm-msp-queue[1166]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 10:40:01 koffee-X406UAR sm-msp-queue[1254]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 10:41:01 koffee-X406UAR sm-msp-queue[1254]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 10:49:53 koffee-X406UAR sendmail[2700]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 10:50:53 koffee-X406UAR sendmail[2700]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 10:50:53 koffee-X406UAR sendmail[2700]: x438orcM002700: from=www-data, size=410, class=0, nrcpts=0, msgid=<201905030850.x438orcM002700@koffee-X406UAR>, relay=www-data@localhost
May 3 11:00:01 koffee-X406UAR sm-msp-queue[3554]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 11:01:01 koffee-X406UAR sm-msp-queue[3554]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 11:20:01 koffee-X406UAR sm-msp-queue[3952]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 11:21:01 koffee-X406UAR sm-msp-queue[3952]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 11:40:01 koffee-X406UAR sm-msp-queue[4384]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 11:41:01 koffee-X406UAR sm-msp-queue[4384]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 11:48:34 koffee-X406UAR sendmail[5128]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 11:49:34 koffee-X406UAR sendmail[5128]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 11:49:37 koffee-X406UAR sendmail[5128]: alias database /etc/mail/aliases rebuilt by koffee
May 3 11:49:37 koffee-X406UAR sendmail[5128]: /etc/mail/aliases: 1 aliases, longest 4 bytes, 14 bytes total
May 3 11:50:49 koffee-X406UAR sm-mta[1173]: restarting /usr/sbin/sendmail-mta due to signal
May 3 11:50:49 koffee-X406UAR sm-mta[1173]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 11:51:49 koffee-X406UAR sm-mta[1173]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
May 3 11:51:49 koffee-X406UAR sm-mta[5217]: starting daemon (8.15.2): SMTP+queueing@00:10:00
May 3 12:40:01 koffee-X406UAR sm-msp-queue[6041]: My unqualified host name (koffee-X406UAR) unknown; sleeping for retry
May 3 12:41:01 koffee-X406UAR sm-msp-queue[6041]: unable to qualify my own domain name (koffee-X406UAR) -- using short name
et mon fichier /etc/hosts à ça :
127.0.0.1 localhost
127.0.1.1 koffee-X406UAR
127.0.0.1 wordpress
127.0.0.1 private
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
J'espère que ça peut aider un peu...